With development of 4G communication, a bandwidth range covered by a radio frequency of a personal terminal product is increasingly wider, which causes bandwidth of a terminal antenna to expand from 824-960 megahertz (MHz) and 1710-2170 MHz to 698-960 MHz & 1710-2690 MHz. For example, an E5 series is required to fully cover all frequency bands of second-generation wireless telecommunications technology (2G), third-generation wireless telecommunications technology (3G), and fourth-generation wireless telecommunications technology (4G), which brings an extreme challenge to the design of an antenna. The design of an antenna needs to break the convention.
Existing terminal devices such as a mobile phone, an E5, and a data card widely use built-in antennas that are in the form of a monopole, an Inverted-F antenna (IFA), a Planar Inverted-F antenna (PIFA), or a Loop. By relying only on radiation of the antennas, bandwidth and coverage of the antennas are limited with a given ground size and a given clearance. On a mobile phone, to resolve a problem of deficiency of low-frequency coverage and high-frequency bandwidth, some antennas featuring tunability are usually designed based on the form of the antennas. In most cases, a solution in which a switch is used together with a variable capacitor or inductor is adopted to achieve a purpose of frequency tuning. For example, a different inductance or capacitance value selected by a switch at a stub of an antenna represents a different load of the antenna, that is, represents a different equivalent electrical length that determines a resonance point of the antenna, as well as a different operating frequency band of the antenna. In a matching position, a different capacitor or inductor is selected by the switch, which leads to a change of antenna matching and a change of the bandwidth and the operating frequency band of the antenna. In this way, an operating state of the antenna is changed by using a switch such that the antenna operates on different frequency bands, thereby achieving a purpose of frequency switching (or tuning).
In the prior art, the purpose of frequency tuning is achieved by using different capacitors or inductors selected by a switch. When frequency tuning is performed by using a capacitor or an inductor, a tunable frequency band range is relatively narrow. Further, because only several capacitors or inductors are generally selected by the switch, frequency bands obtained in such a tuning manner are discontinuous.
Further, a solution with a switch-gated capacitor or inductor in the prior art is generally applied to a mobile phone. However, because different terminals are in different forms, the solution with a switch-gated capacitor or inductor that is applicable to tuning of a mobile phone is not applicable to other terminals. Using a Wide-Area Network (WAN) card as an example, a mobile phone and a WAN card have different ground lengths, and the ground length of the former is longer than the ground length of the latter by more than 50 millimeters. Therefore, when the solution with a switch-gated capacitor or inductor that is applicable to a mobile phone is applied to a WAN card, a shorter ground length of the WAN card deteriorates low-frequency performance of the antenna.
Further, when the solution with a switch-gated capacitor or inductor is used for frequency tuning, an insertion loss of the switch is great, and impedance between the switch and the tunable antenna is prone to mismatch.